The gas, which is emitted in the paddy fields and increases the earth's temperature is

Question

UnlockIAS · Accepted Answer

Correct — D, Methane. A transplanted paddy field is kept under standing water, and waterlogged soil quickly runs out of oxygen. In those anaerobic conditions methane-producing archaea (methanogens) decompose the organic matter in the soil and release methane (CH4). The gas escapes to the atmosphere in three ways — bubbling up through the water (ebullition), diffusing across the water surface, and, most of all, travelling up the aerenchyma, the air channels inside the rice plant itself. Methane is a potent greenhouse gas: over a hundred years one tonne of it traps roughly 28-30 times as much heat as a tonne of carbon dioxide (IPCC AR5 global warming potential). That is why flooded rice cultivation is counted among the largest agricultural sources of global warming, alongside enteric fermentation in cattle.

Why the other options are wrong — (a) Nitrogen gas (N2) already makes up about 78% of the atmosphere and is not a greenhouse gas at all — it is a symmetric diatomic molecule whose vibrations do not change its dipole, so it cannot absorb infrared radiation. Do not confuse it with nitrous oxide (N2O), a genuinely potent greenhouse gas released from nitrogen-fertilised soils; N2O is a different molecule and is not what this option says.  (b) Carbon dioxide is a greenhouse gas, but it is not what distinguishes a paddy field. The signature of a flooded, oxygen-free paddy soil is methanogenesis; the growing rice crop is in fact absorbing CO2 by photosynthesis. The question asks for the gas characteristically emitted from paddy fields, and the key marks methane.  (c) Carbon monoxide is a toxic pollutant from incomplete combustion (vehicles, stubble burning, cooking fires). It is not a significant direct greenhouse gas — it affects climate only indirectly, by using up hydroxyl radicals and so prolonging the life of methane — and it is not a characteristic paddy-field emission.

Concept: Methane is produced by anaerobic decomposition — the breakdown of organic matter by microbes where there is no free oxygen. Any waterlogged, organic-rich environment therefore emits it: natural wetlands and marshes, flooded rice paddies, the rumen of cattle and buffaloes, landfill sites, and the flooded reservoirs behind dams. Coal mines and oil-and-gas systems add methane of geological origin. Methane's atmospheric lifetime is only about a decade — far shorter than CO2's — but its warming punch per tonne is far greater, which makes it a 'short-lived climate pollutant' and an attractive target for quick climate action.

State PCS 2019 Q27 — Environment & Ecology PYQ with Answer

State PCS 2019 Q27 — Environment & Ecology PYQ with Answer